1. Field of Invention
This invention relates to a method and apparatus for achieving ultra high density integrated circuit packages incorporating a plurality of ultra-thin molded integrated circuit packages stacked and interconnected into an ultra-high density three-dimensional module.
2. Discussion of the Related Art
Packaging techniques for integrated circuits have been developed in the past in an attempt to satisfy demands for miniaturization in the semiconductor industry. Improved methods for miniaturization of integrated circuits enabling the integration of millions of circuit elements into single integrated silicon embodied circuits, or chips, have resulted in increased emphasis on methods to package these circuits in space efficient, yet reliable and mass producible packages.
The introduction of highly sophisticated integrated circuit microprocessors led to the rapid development of complex personal computers and other common bus systems utilizing a variety of integrated circuit elements such as memory devices (DRAMS, SRAMS), programmable logic arrays (PLAs), microprocessors (CPUs), coprocessors, and other related integrated circuit elements which had to be assembled, mounted and interconnected into as compact, yet reliable packages as feasible to satisfy the industry demands for miniaturization.
Other key considerations in developing packaging for such circuits have been the cost of manufacture, the reliability of the packaged device, heat transfer, moisture penetration, standardization of mounting and interconnect methods, and the ability to test and control the quality of the packaged devices.
In the past, one area of concentration for high density packaging has been memory devices such as SRAMS and DRAMS. Prior systems typically utilized a transfer molded plastic encasement surrounding the integrated circuit and having one of a variety of pin out or mounting and interconnect schemes. The older M-DIP (Dual-In-Line-Plastic) provides a relatively flat, molded package having dual parallel rows of leads extending from the bottom for through-hole connection and mounting to an underlying printed circuit board. These packages provided 100 mil spacing between leads.
A more dense package was the 100-mil SIP (Single-In-Line-Plastic) which was assembled on edge with two rows of 100-mil staggered leads extending from the bottom edge for through hole assembly. Another popular prior art package is the PLCC (Plastic Leaded Chip Carrier) SOJ (Small Outline J-leaded) molded package with twenty surface-mount designed J-leads (length 0.67", width 0.34", height 0.14"). This prior art package is illustrated schematically in FIG. 1 and shown at approximate actual size in FIG. 2.
In order to obtain more density and provide lower cost socketability (i.e. removable mounting) and to allow for after-market sale of additional memory units the SIMM (Single-In-Line Memory Module) was developed. This package is schematically illustrated in FIG. 3. In this package typically nine one megabyte or four-megabyte DRAMS are surface mounted into a socket which is in turn edge-mounted on a large printed circuit board containing additional sockets or components. While this design provided some increase in density, it had the drawback of providing a module extending from one-half to nearly two inches vertically above the printed circuit board.
Newer, higher density versions of the SIMM design with even small versions of the DRAM plastic package have been developed. These thinner versions of SOJ DRAMS are one half the thickness (having a plastic packaging thickness of about 70 mils) of standard SOJ designs, and have been mounted on both sides of printed circuit boards. Even smaller TSOP packages have been developed experimentally with a plastic thickness of one millimeter and lower profile gull-wing leads for surface mounting. FIGS. 1-3 illustrate typical embodiments of some of these prior art packages. Based on experience with those prior art designs, for reasons of reliability related to moisture penetration and mechanical integrity the industry has adopted a standard thickness for plastic packaging of approximately one millimeter (40 mils), or approximately 10.5 mils on each side of a 19 mil thick integrated circuit element.
In contrast to such prior art systems, the packaging method of the present invention provides a reliable, cost efficient, easily manufacturable package with a plurality of ultra-thin level-one package elements assembled in an integrated module or level-two package which can be mounted to a printed circuit board directly or via an underlying socket or header.